The present invention is directed in general to a system for transmitting digital information on transmission media and, more particularly, to a balanced differential line driver circuit and method for driving separate conductor pairs in a transmission line or cable with a multi-level digitally encoded line driving signal.
The transmission of digital information over two-conductor balanced transmission cable, such as over twin-axial or flat-line cable or over an unshielded twisted pair telecommunication system cable, is well known. The ability to successfully transmit digital information over such a cable and recover it at a receiving end is dependent on controlling certain aspects of the transmission system. For example, successful data transmission and recovery is dependent on minimizing the introduction of noise and signal distortion into the transmitted data signal. A poorly designed driver can cause noise and distortion of the transmitted data signal and thereby causes errors in the detection of the transmitted data. The most significant form of noise introduced into the transmitted signal is common mode noise.
One source of common mode noise is the unbalanced driving of the two-conductor cable by the line driver apparatus. As used herein, balanced driving of two-conductor cable is ideally achieved when the two conductors of the cable are differentially driven such that the signal potentials on the respective conductors are identical in absolute magnitude, relative to a baseline reference, but of opposite polarity at each point in time. As a result, there is a zero net direct current or unbalanced signal carried by the two conductors during balanced driving of the cable. Correspondingly, unbalanced driving of the two-conductor cable connotes that the two conductors are differentially driven in an unbalanced fashion such that there is a non-zero net signal carried by the two conductors over time. The non-zero net signal represents common mode noise.
A second source of common mode noise is crosstalk between proximate transmission cables. The two-conductor cable discussed above typically represents only one of multiple unshielded insulated pairs of conductors within a single multipair cable. Crosstalk results primarily from capacitive interpair coupling from one conductor pair into another proximate conductor pair. The induced differential signals represent the crosstalk noise. Thus, crosstalk is also a source of transmission interference. A major source of emissions in two-conductor cable is common mode noise since such noise is a non-zero net signal carried by the two-conductor media. Thus, unbalanced driving of the transmission media is also a cause of spurious emissions.
A third source of common mode noise in differentially driven two-conductor cable is the difference in signal edge rates, i.e., rising and falling edge rates, on the respective conductors, since such differences do not mutually cancel. The difference in edge rates can result in common mode noise due to the instantaneous unbalanced driving of the two-conductor cable.
Successful data transmission and recovery also depends on minimizing the return loss of the line driver apparatus. Return loss is caused by a mismatch between the line driver output impedance and the impedance of the transmission cable. Such impedance mismatch results in signal reflections at the interface between the line driver and the transmission cable. The signal reflections cause phase addition and cancellation of the signals being transmitted which results in signal distortion and jitter.
A further parameter upon which successful data transmission and recovery depends is the baseline wander of the transmitted signal. Baseline wander represents the drifting of a reference level, e.g., an average DC voltage level, with respect to which the logic levels of the transmitted signal are defined, and represents another form of common mode noise. Various data encoding schemes such as Manchester encoding and modified duobinary encoding serve to minimize and substantially eliminate baseline wander. As known in the art, such encoding schemes require transmission of a multi-level signal having multiple possible signal levels which are symmetrically centered around a DC voltage level.
Line driver apparatus, as known in the art for driving a pair of conductors, is typically constructed to have a pair of voltage source drivers each with its own internal impedance. As a result, the respective impedances of the drivers of a pair of voltage source drivers may not be identical. A difference between respective impedances of the drivers, over the differential signal swing, results in different rising and falling edge rates of the transmitted signal. As noted above, the difference in edge rates is a sources of common mode noise. A difference between the driver impedances also results in the pair of voltage source drivers driving the pair of cable conductors to different voltage levels for positive and negative signals, which contributes to an unbalanced condition, thus causing baseline wander and common mode noise.
Also, in the case where voltage source drivers are provided as emitter follower circuits, the emitter follower circuits do not drive the conductor pair with a constant source impedance for all possible output signal voltage levels. As a result, a mismatch between source and cable impedances intermittently occurs during transmission and causes the above-noted problem of return loss. Also, emitter follower circuits inherently drive a transmission conductor with faster rising edge rates than falling edge rates. This results in significant differences in signal edge rates across a differentially driven two-conductor cable thus contributing to common mode noise.
The continuing trend with respect to digital information transmission is to increase the effective bandwidth of the transmission media by sending data at a faster data rate. However, there are limitations on the ability to increase the data rate of transmissions over two-conductor transmission cable, especially unshielded two-conductor cable such as used in telecommunication systems. One limitation on increasing the data rate is that spurious emissions increase from unshielded cable as the data rate increases. For example, the increased data rate represents an increased rate at which signal edges occur, so that noise generated by edge rate differences increases with the data rate.
Also, an increased data rate represents an increase in the high frequency spectral content of the transmitted signal. This high frequency spectral content is, in turn, a source of electromagnetic emissions from the transmission media which, as noted above, are a cause of crosstalk and common mode noise. Further, the levels of freely propagating electromagnetic emissions resulting from transmission of digital information are subject to conformance with federal and international standards, for example the standards established by the Federal Communications Commission. Therefore, such standards can impose limits the data rate.
One solution known in the art for reducing emissions associated with high frequency spectral content is the use of an encoding scheme such as the above-noted modified duobinary encoding, which has the effect of reducing the high frequency spectral content of the transmitted signal. However, the emission-reducing effects of such an encoding scheme, which requires driving a two-conductor cable with a multi-level signal, will be significantly impaired if the line driver apparatus cannot drive the cable in a tightly controlled balanced fashion.
Another solution known in the art for reducing emissions associated with high frequency spectral content is the use of shielded cable. However, disadvantageously, such cable is more expensive than unshielded cable and not as readily available as an installed base media to be exploited.
A further solution known in the art for avoiding the problem of emissions associated with high data transmission rates is the optical encoding of the signals and transmission of the optically encoded signals over optical fiber cable. To this end, ANSI standard X3T.9 was developed to standardize parameters of such digital transmission over optical fiber media and defines the fiber data delivery interface (FDDI). The specific characteristics of the FDDI are known to those skilled in the art and are not described herein. However, it is noted that the FDDI dictates a particular encoding scheme for data transmission to minimize baseline wander and a data rate of 125 Mbit/seconds for an NRZI (non-return-to-zero-inverted) coded signal.
One disadvantage of optical encoding is the significant expense associated with apparatus required to translate into optical form the binary data in electrical form on each of a plurality of different data channels.